There are a number of things Mr.Ehrlich discovered..
1) Postulated the existence of the blood-brain barrier.
2) Worked with dyes that enabled various bacteria to be stained,and therefore seen with ease.His work was a precursor to the "Gram-staining" technique that place bacteria into either Gram-negative or Gram-positive categories.
3) Worked on his "cure" for syphilis,the "magic bullet, Salvarsan (a.k.a. Formula 606)-This was in 1910...
Although he was by profession, a chemist,he made valuable biological discoveries..
Now,we all know who is a "close cousin" to syphilis by now (borrelia),and a recent post here by emla999/Lyme regarding Methylene Blue(a dye) shows that it can cross the blood-brain barrier to possibly treat Parkinson's /Alzheimer's in microscopic quantities..